The proposed research concerns the synthesis of a series of fraudulent nucleosides in which a phosphonamido moiety has been substituted for a trigonal ring carbon in the purine or pyrimidine base. These compounds will be tested as inhibitors of the appropriate nucleoside deaminases, in order to evaluate this strategy of synthesizing transition state analogs for enzyme-catalyzed reactions which proceed via tetrahedral intermediates. The target compounds, which are members of hitherto unknown heterocyclic ring systems, may have synergistic activity with clinically important antineoplastic drugs as well as antiviral or cytostatic activity in their own right.